This invention concerns a method for the continuous casting of peritectic steels.
By peritectic steels are meant steels with a carbon content between 0.10%. and 0.15% and at times between 0.09% and 0.16%.
The method of this invention is applied to the field of the production by continuous casting of thin slabs of special steels having high mechanical and technological properties.
By thin slabs are meant slabs with a thickness less than 90 mm. to 95 mm. and a width between 800 mm. and 2500 mm. to 3000 mm.
The method according to the invention has the purpose of reducing all the characteristics of defects and surface irregularities and also of great sensitiveness to cracks and depressions which have so far not permitted a use of peritectic steels on a large scale with satisfactory qualitative results.
Peritectic steels, that is to say, those steels which have a low carbon content between 0.10% and 0.15%, even though the range is sometimes enlarged to 0.09% to 0.16%, possess a plurality of metallurgical characteristics which are derived from their composition and which make very delicate the casting process if it is desired to obtain good qualitative results.
A typical fault encountered in these steels is the presence of surface irregularities and depressions, this presence being particularly accentuated in the case of peritectic steels with a carbon content between 0.10% and 0.13%.
This type of defect is mainly caused by the allotropic conversion in the cooling phase and, in particular, between 1493.degree. C. and T'.
The temperature of 1493.degree. C. is the peritectic temperature at which the nucleation and growth of the gamma phase of composition J (with a carbon content of 0.15%) begin from the liquid of composition B (with a carbon content of 0.51%) and from the solid delta phase of composition H (with a carbon content of 0.10%).
This conversion continues at a constant temperature until the complete disappearance of the liquid phase and until complete solidification with a final presence of the two delta and gamma phases.
With the cooling proceeding below 1493.degree. C., there takes place a continuous conversion of delta phase into gamma phase until there is only gamma phase at the temperature T'.
FIG. 1 shows the upper lefthand end of the Iron-Carbon diagram from which are deduced the above solidification methods.
Therefore, in the temperature gap between 1493.degree. C. and T', the delta phase being converted into the gamma phase undergoes a change of lattice from the body-centred cubic lattice (CCC) to the face-centred cubic lattice (CFC).
This change of lattice causes a resulting accentuated thermal shrinkage different from that of the rest of the solid solution (gamma phase).
The differentiated shrinkage leads to a strong tendency towards non-uniformity and surface irregularities and depressions.
The peritectic steels also have, to a certain extent, a rather great sensitiveness to cracks.
This characteristic is found in peritectic steels with a carbon content close to the upper limit of such steels, and even beyond that limit, and therefore is not restricted to peritectic steels alone.
This sensitiveness to cracks is a metallurgical result of the fact that these steels have a strong tendency towards the formation of depressions and, therefore, tend to have a structure of first solidification with irregular austenitic grains of great dimensions and a resulting reduction of ductility in the hot state.
All these problems of a metallurgical nature have so far prevented the continuous casting of peritectic steels and have forced the producers to avoid the typical range of these steels (0.10% to 0.15%) and to try to obtain analogous mechanical properties with corrections of the percentages of composition of other components such as manganese, silicon, etc.
The article "Gallatin Steels follow thin slab route" in the Trade Journal "Iron and Steel International" of 1994 states clearly on page 55 and the following pages that no one has so far been able to cast peritectic steels continuously; the table given on page 57 also shows clearly the absence of such types of steels.
At the Conference held in Peking in September 1993 a report entitled "Near-Net-Shape-Casting" was presented and was shown on page 391 and the following pages of the documents of the Conference.
That report indicates what was confirmed thereafter in the aforesaid article in the "Iron and Steel International".
This shows that technicians have been seeking for a long time a method suitable to cast continuously, and advantageously in the form of thin slabs, peritectic steels, but without yet having succeeded.